As a power converter to convert AC power from AC power supply to DC power, there is a two-stage converter including a PFC converter and an isolation type DC-DC converter or a single-stage converter having operation of a PFC converter and an isolation type DC-DC converter integrated. A single-stage converter has a smaller number of converter stages or diagram components than a two stage converter, so a single-stage converter is suitable for achieving low cost and high efficiency. Such examples are shown in FIG. 9 in JP2003-23775 A and in FIG. 5 in JP2008-289228 A.
In JP2003-23775 A, there are semiconductor switching components Q1 through Q3 in FIG. 9, out of which Q2 and Q3 operate complementarily to Q1. Technique is disclosed to control a terminal voltage of a condenser (capacitor) C1, that is, an intermediate voltage is controlled at a constant setting value by changing on-off duration ratio of switching frequency to control semiconductor switching components Q1 through Q3, a terminal voltage of a condenser (capacitor) C3 or output voltage is controlled at a constant setting value by changing on-off cycle of switching frequency to control these semiconductor switching components.
In JP2008-289228 A, control electrode of semiconductor switching components Q1 through Q3 in FIG. 5 can be controlled individually, and technique is disclosed to control power factor and output voltage.
Even though control method disclosed in JP2003-23775 A can control an intermediate voltage at constant setting value, usage frequency expands to low side and size of used diagram components, especially core size, needs to be larger to lose compactness with high cost, when load current is high with high usage power. There is an additional problem of larger loss at rated load, because discontinuous mode is used for power factor.
On the other hand, a control method disclosed in JP2008-289228 A applies a continuous mode for power factor correction, in order to avoid loss at rated load with discontinuous mode, but there is a problem that an intermediate voltage fluctuates and goes up at light load, requiring condensers with high breakdown voltage, because it does not control a terminal voltage of a condenser C1 or an intermediate voltage at constant setting value. This results in higher cost. Additionally input current waveform is not corresponding to the change of AC power supply voltage in the vicinity of zero, and power factor is not improved.
The present invention is intended to solve those problems and provide a low-cost and high-efficiency power converter.